48 research outputs found
Role of dipolar and exchange interactions in the positions and widths of EPR transitions for the single-molecule magnets Fe8 and Mn12
We examine quantitatively the temperature dependence of the linewidths and
line shifts in electron paramagnetic resonance experiments on single crystals
of the single-molecule magnets Fe and Mn, at fixed frequency, with
an applied magnetic field along the easy axis. We include inter-molecular
spin-spin interactions (dipolar and exchange) and distributions in both the
uniaxial anisotropy parameter and the Land\'{e} -factor. The temperature
dependence of the linewidths and the line shifts are mainly caused by the
spin-spin interactions. For Fe and Mn, the temperature dependence of
the calculated line shifts and linewidths agrees well with the trends of the
experimental data. The linewidths for Fe reveal a stronger temperature
dependence than those for Mn, because for Mn a much wider
distribution in overshadows the temperature dependence of the spin-spin
interactions. For Fe, the line-shift analysis suggests two competing
interactions: a weak ferromagnetic exchange coupling between neighboring
molecules and a longer-ranged dipolar interaction. This result could have
implications for ordering in Fe at low temperatures.Comment: published versio
Nuclear spin-lattice relaxation in ferrimagnetic clusters and chains: A contrast between zero and one dimensions
Motivated by ferrimagnetic oligonuclear and chain compounds synthesized by
Caneschi et al., both of which consist of alternating manganese(II) ions and
nitronyl-nitroxide radicals, we calculate the nuclear spin-lattice relaxation
rate 1/T_1 employing a recently developed modified spin-wave theory. 1/T_1 as a
function of temperature drastically varies with the location of probe nuclei in
both clusters and chains, though the relaxation time scale is much larger in
zero dimension than in one dimension. 1/T_1 as a function of an applied field
in long chains forms a striking contrast to that in finite clusters, diverging
with decreasing field like inverse square root at low temperatures and
logarithmically at high temperatures.Comment: to be published in Phys. Rev. B 68 August 01 (2003
Magnetic Field Effects on the Far-Infrared Absorption in Mn_12-acetate
We report the far-infrared spectra of the molecular nanomagnet Mn_12-acetate
(Mn_12) as a function of temperature (5-300 K) and magnetic field (0-17 T). The
large number of observed vibrational modes is related to the low symmetry of
the molecule, and they are grouped together in clusters. Analysis of the mode
character based on molecular dynamics simulations and model compound studies
shows that all vibrations are complex; motion from a majority of atoms in the
molecule contribute to most modes. Three features involving intramolecular
vibrations of the Mn_12 molecule centered at 284, 306 and 409 cm-1 show changes
with applied magnetic field. The structure near 284 cm displays the
largest deviation with field and is mainly intensity related. A comparison
between the temperature dependent absorption difference spectra, the gradual
low-temperature cluster framework distortion as assessed by neutron diffraction
data, and field dependent absorption difference spectra suggests that this mode
may involve Mn motion in the crown.Comment: 5 pages, 4 figures, PRB accepte
Detailed single crystal EPR lineshape measurements for the single molecule magnets Fe8Br and Mn12-ac
It is shown that our multi-high-frequency (40-200 GHz) resonant cavity
technique yields distortion-free high field EPR spectra for single crystal
samples of the uniaxial and biaxial spin S = 10 single molecule magnets (SMMs)
[Mn12O12(CH3COO)16(H2O)4].2CH3COOH.4H2O and [Fe8O2(OH)12(tacn)6]Br8.9H2O. The
observed lineshapes exhibit a pronounced dependence on temperature, magnetic
field, and the spin quantum numbers (Ms values) associated with the levels
involved in the transitions. Measurements at many frequencies allow us to
separate various contributions to the EPR linewidths, including significant
D-strain, g-strain and broadening due to the random dipolar fields of
neighboring molecules. We also identify asymmetry in some of the EPR lineshapes
for Fe8, and a previously unobserved fine structure to some of the EPR lines
for both the Fe8 and Mn12 systems. These findings prove relevant to the
mechanism of quantum tunneling of magnetization in these SMMs.Comment: Phys. Rev. B, accepted with minor revision
Deuteron NMR of a manganese-ion cluster nanomagnet
The temperature dependence of deuteron NMR spectra of the fully deuterated manganese ion
cluster nanomagnet -acetate has been measured. Two sets of deuteron peaks have been
observed which both shift towards lower fields with decreasing temperature. The strongly
shifted peak is associated with the deuterons of the four molecules which are directly
attached to the Mn core. The less shifted peak is due to the deuterons, which are three
bonds away from the oxygen that is attached to Mn ions, as well as to the solvated and
molecules. The results imply that the unpaired spin density extends over the whole
molecular cluster rather than being concentrated at the Mn ions